The aim is to reduce the signature of e.g. military vehicles using stealth technique. This is achieved, inter alia, by the vehicles being as far as possible prevented from emitting compromising emanations from, for instance, hot surfaces or emitted hot gases and from reflecting energy, for instance, from an enemy's reconnaissance equipment. The surveillance radiation may be microwave radiation, such as radar or laser radiation.
The heat radiation, which can be detected by IR sensors, can be reduced by the hot gases being cooled before being emitted. As a result, also the temperature of the surfaces associated with the exhaust is usually reduced. This is carried out according to prior-art technique, for instance, by fresh air from the exterior being added to the exhaust gases before being emitted. The effect of the exhaust on the heat signature of the vehicle highly depends on the difference between the heat radiation from the exhaust, i.e. from the surfaces of the exhaust as well as the emitted hot gases, and the parts of the vehicle which surround the exhaust. Attempts are made to reduce this difference to such an extent that the exhaust cannot be localised on a sensor image of the vessel and preferably to such an extent that the exhaust does not add to the signature of the vessel.
The reflection of radar energy can be reduced significantly according to the above-mentioned stealth technique, usually by the vehicle absorbing the received radiation or letting as much as possible thereof be reflected in a predetermined direction which is different from the direction to the sender. The absorption can be provided by special absorbing materials on the surfaces of the vehicle. The reflection can be controlled by the vessel being made with plane surfaces which are electrically conductive and by not having cavities with inner corners that cause scattered reflection of the received radiation. Without this signature adaptation, the radiation is reflected in an uncontrolled manner in a large polar angular range and will, depending on the character of the vehicle, be reflected to a varying degree to the location of transmission. A particularly great amount is reflected if the vehicle happens to comprise areas formed as corner reflectors.
In the known exhausts, the reflecting radar energy is usually not taken into consideration. In most cases, they have naked cavity-forming outlets even if they are signature adapted to IR radiation. A common known embodiment is an exhaust pipe which is surrounded by a pipe to which fresh air is supplied. The supply of this air can be driven by means of an electric fan or by the exhaust gases acting as a drive gas in an outlet part formed as an ejector.
The drawbacks of the known solutions to the problem are that                they do not sufficiently consider the problems with the compromising emission of heat and/or the reflection of radar energy        they often utilise complicated equipment, such as electric fans, which makes the devices complicated and expensive to manufacture and maintain as well as vulnerable in a fighting environment.        